Because of the limited energy storage capacity of conventional electrochemical batteries, there have been various attempts at developing batteries powered by radioactive elements, due to the higher theoretical limits on the energy density from such radioactive elements or radioisotopes. The most common type of nuclear batteries are known as radiothermal generators (RTGs), which utilize the heat produced when the decay energy of the radioactive material is absorbed by the battery material to produce power. Such batteries are commonly used in navigation buoys, weather stations, various space based applications such as satellites, and also for nuclear powered pacemakers. For such applications, the most commonly used radioisotopes are strontium-90 and plutonium-238, although cesium-137 and curium-242 and curium-244 can also be used.
Also known are nuclear batteries which utilize an indirect conversion approach. In such indirect conversion devices, a substrate material impregnated with a radioisotope and a phosphor powder is sandwiched between two photovoltaic cells. The decay particles emitted by the radioisotope excite the phosphors, causing light to be emitted, which is then absorbed by the photovoltaic cells, generating electricity. The potential applications of these devices are limited by the relatively low conversion efficiencies and poor stability of the luminescent material, due to radiation damage.
U.S. Pat. No. 4,024,420 to Anthony et al., describes a further type of nuclear battery, namely a deep diode atomic battery made from a bulk semiconductor crystal powered by gamma rays and x-ray emission from a radioactive source embedded in a central cavity in the interior of the semiconductor crystal. As the radioactive source of this device is stated to be preferably a high energy source, the energetic emission from the radioactive source can lead to radiation damage and heating of the bulk semiconductor crystal, with consequent lowered efficiency and shortened operating lifetime.
U.S. Pat. No. 5,260,621 to Little et al. discloses a further solid state nuclear battery, comprising a relatively high energy radiation source, such as promethium-147, and a bulk crystalline semiconductor which is characterized by defect generation in response to the radioisotope. The materials of the semiconductor are chosen such that the radiation damage is repaired by annealing in real time at the elevated operational temperature of the battery. This device has several shortcomings, due to the inherent inefficiency of the semiconductor used, which necessitates the use of a high energy radiation source. As noted above, such a source can produce severe lattice damage, requiring that the material be self-annealing, to achieve an acceptable carrier lifetime and output. As continuous annealing of the semiconductor is required, the useful life of such a battery is limited.
U.S. Pat. No. 5,396,141 to Jantz et al. discloses a further solid state nuclear battery, which utilizes a radioactive source associated with a p-n junction. In this device, the semiconductor material includes integrated circuitry formed therein. Because of this, physical separation of the nuclear battery from the integrated circuits is required to protect against the effects of thermal degradation and radiation damage, from both chronic radiation exposure and damage due to a single high-energy event, which is of particular concern with the high-energy sources described. Further, because of the physical separation of the battery from the electronic circuits, the potential for miniaturization and incorporation of this device into integrated circuit applications is limited.